In many systems, females exhibit a strong, localized immune response to the male's ejaculate that has traditionally been viewed as a defense against sexually transmitted disease. However, the female response has been noted to kill sperm in some systems. In systems with intense sperm competition (i.e. systems where multiple ejaculates reside in the female's reproductive tract at any given time), males may interfere with the female's immune response in order to increase their sperm viability, thereby increasing their chance of fertilization success over rival males. To address this possibility, genes that code for female immune proteins and male ejaculate proteins will be manipulated in order to elucidate their potential effect on mating behavior, sperm viability and sperm competition. Specifically, three objectives will be addressed including (1) if the female immune response to insemination underlies the reduction in sperm viability commonly found among invertebrates, (2) if male fertilization success is constrained by the female immune response, and (3) the consequences of the male ejaculate on female immunity.
The experimental design takes a comprehensive approach, examining numerous genes associated with a variety of physiological systems. Considering that reproduction and immunity are both highly conserved biological systems, there appears to be great potential for such an interaction to operate in a wide array of sexually reproducing systems. Thus, the immune system may play a key role in the evolution of mating systems and visa versa. The proposed research provides an ideal opportunity to promote scientific training and education. Specifically, the project provides independent research opportunities to undergraduate and graduate students interested in evolutionary biology, ecological immunity and learning molecular techniques.
Recent evidence in the fruit fly Drosophila melanogaster suggests that sperm death in the female reproductive tract is extensive (~70% of the initial viable ejaculate) and likely due to the direct actions of the female. Elucidating the underlying mechanism for such extensive mortality in a model biological system may provide broad-reaching insights into our understanding of basic reproductive biology. We hypothesized that sperm death may be due to a female immune response to mating, considering that such a response was recently discovered in invertebrates and has been shown to influence sperm viability in vertebrates. This immune response likely evolved to defend against sexually transmitted pathogens, but may kill sperm via unintended collateral damage of the immune effectors. In short, our main objective was to determine the extent of sperm death and to elucidate the underlying mechanism(s), which potentially include (1) female age, (2) rival ejaculates, (3) sperm senescence, and (4) an acute female immune response to mating. The implementation of the grant was highly successful. We made several exciting discoveries that change our view of reproduction in this model system and potentially in the majority of animals. First, sperm death in the reproductive tract of females is not excessive, as previously reported. The minimal death uncovered here suggests that stored sperm viability is sustainably high and should have a minimal influence on the evolution of female remating rates. Second, systemic immune activation dramatically decreases sperm viability in both sexes, and that this effect is not the result of a resource trade-off between immune and reproductive systems. Instead, our data are consistent with collateral damage from immune effectors. Third, females can detect and purge contaminated ejaculates from sperm storage. This creates the opportunity for antagonistic coevolution between female immune systems and male ejaculates, as well as influence male sperm competition if infections are common. Fourth, we show that males modify the ratio and amount of seminal fluid proteins when under intense sperm competition, while sperm production remains constant. This is a significant departure from contemporary theoretical and empirical work addressing sperm competition, which largely suggests that sperm number is modified not ejaculate proteins. To date, this proposal has produced 3 published papers, 1 submitted manuscript and 3 more manuscripts in preparation. Additionally, this grant has supported the training of 2 post-doctoral researchers, 1 graduate student, 12 undergraduate students, 1 high school student and 1 high school teacher. In total, these trainees and I have given 15 presentations at local, domestic and international meetings.
